Means for and method of controlling the generation of x-rays



NOV. 25, 1958 CUMMlNGs 2,862,107

MEANS FOR AND METHOD OF CONTROLLING THE GENERATION OF X-RAYS Filed April 6, 1951 2 Sheets-Sheet 1 HAROLD R. CUMMINGS Nov. 25, 1958 H. R. CUMMINGS MEANS FOR AND METHOD OF CONTROLLING THE GENERATION OF X-RAYS Filed April 6, 1951 FIG. 9

2 Sheets-Sheet 2 FIG. IO

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GRID-CATHODE VOLTAGE MEANS FOR AND METHQD OF CONTROLLING THE GENERATION F X-RAYS Harold R. Cummings, Waterford, Wis., assignor to General Electric Company, a corporation of New York Application April 6, 1951, Serial No. 219,660

8 Claims. (Cl. 250--99) The present invention relates in general to electronics, and has more particular reference to the control of electron movement in electron flow devices, the invention more especially pertaining to the control of electrons flowing in X-ray generating devices, and providing for the control of such devices in novel, simple and exceedingly efiicient fashion.

Electron flow devices of the character herein contemplated comprise an electron emission element forming a cathode, and a cooperating anode comprising an electron target, the anode and cathode being normally enclosed v and hermetically sealed within a suitable envelope to form a so-called tube. The device or tube is operated by energizing the cathode for electron emission and causing the emitted electrons to travel to and to impinge upon the anode, at the electron target surface thereof. Electrons emitted at the cathode are thus caused to move toward the anode under the influence of electrical potential applied between the anode and cathode, electron traveling speed between cathode and anode being a function of the driving potential applied therebetween. Electron flow devices, having inherent self rectifying characteristics, may be operated either by application of unidirectional electron driving potential between the anode and cathode of the device, inwhich case electrons are driven continuously from cathode to anode, where the driving potential is constant; or the device may be operated by applying full wave rectified potential, between anode and cathode,

in which case electron flow approximately follows unidirectional potential fiuctuations; or the device may be operated by applying potential of alternating character, between anode and cathode, in which case electron flow will take place only during the half cycle intervals of driving potential applied in one direction, no flow occurring during half cycle intervals of potential applied in the opposite direction.

X-ray generators which comprise specialized electron flow devices function to produce X-rays at the anode target surface of the device as the result of impingement thereon, at high speed, of electrons emitted at the cathode. The X-rays so generated pass from the target through the envelope of the tube in the form of an X-ray beam for application to useful purposes outside of the envelope.

The quantity of X-rays generated at the anode of an X-ray tube is a function not only of the electron driving potential maintained between the anode and cathode of the tube for electron driving purposes, but is also a function of the rate at which electrons are emitted by the cathode. The intensity of the X-rays, that is to say, their penetrating ability, is a function of the electrical potential applied between the anode and cathode for electron driving purposes. Accordingly, it is the common practice to control the character of the emitted X-ray beam, produced at the anode by the operation of the device, by adjusting the rate of electron emission at the cathode or the electron driving potential between anode and cathode, or both.

The control of the electron driving potential may be nited States Patent 0 2,862,107 Patented Nov. 25, 1958 and commonly is accomplished by means of suitable equipment in the electrical power supply system used for the operation of the device. Control of electron emission at the cathode likewise may be and commonly is accomplished by means of suitable conventional control devices, the cathode usually comprising a filament adapted to emit electrons when excited electrically, the electron emission rate being proportional to the degree of cathode excitation, usually measured in terms of the electrical current applied to the cathode filament for exciting purposes. Thus it is that the emitted X-ray beam is conventionally regulated and its character defined in terms of cathode exciting current, usually expressed in amperes, and the electron driving potential, applied between anode and cathode, is usually expressed in terms of voltage.

It is desirable that the sectional configuration of the emitted X-ray beam be uniform throughout the operating voltage range of the generator, and it is further desirable that the boundaries of the beam be sharply defined, and that X-ray intensity be uniform throughout the beam. These factors, in turn, are determined by the uniformity of electron impingement within the impinging area of the anode target surface, and the sharp definition of said electron impinging area. It is also desirable to operate the device in such fashion as to avoid excessive target loadings, by keeping the rate of electron impingement, per unit area of the target, Within safe limits. Accordingly, for optimum conditions of operation, in an X-ray generating tube, it is desirable to control the electron stream between cathode and anode in such fashion as to obtain electron impingement upon the anode target within a sharply defined impingement area thereof, and substantially uniformly throughout such area, throughout the voltage range at which the device is intended to operate.

The present invention has for an important object the provision of simple and inexpensive yet eflective means, in an electron flow device, for controlling the rate of electron flow between the cathode and anode; a further object being to provide for such electron flow control, in an X-ray generating tube, directly, by means of a flow control grid disposed in the electron stream, thereby obviating the necessity of obtaining control through adjustment of cathode excitation.

Another important object is to provide for the control of an X-ray generator tube by means of an electron flow control grid disposed between the anode and cathode of the tube; a further object being to provide improved and simplified means for biasing the grid of an electron flow device with respect to the electron emitting cathode thereof.

Another important object is to provide an improved circuit system for applying a bias to an electron flow control grid; a further object being to provide a system for supplying an alternating or fluctuating bias on the grid in timed relation with respect to the fluctuations in the electron driving potential in a self-rectifying electron flow device, a still further object being to employ an insulation transformer in the grid biasing power supply system.

Another important object is to provide an X-ray tube having electron flow control means comprising a grid interposed between the anode and cathode of the tube for the purpose of controlling the beam of electrons to cause the same to impinge upon the anode target of the tube within a sharply defined area thereof; a further object being to control the size of said impingement area by adjusting the bias potential applied to the grid; a still further object being to provide for automatically adjusting the size of the impingement area as a function of target loadings to maintain the target loading within useful and safe limits, by automatically regulating the size of the impingement area to that which affords optimum detail in X-ray shadow pictures, produced by application of the resulting X-ray beam to picturing purposes, without overloading the target.

The foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawings, discloses a preferred embodiment of the invention.

Referring to the drawings:

Fig. 1 is a sectional view through an X-ray generating tube embodying the present invention;

Figs. 2, 3 and 4 are sectional views, respectively taken substantially along the lines 2-2 in Fig. 1, 3-3 in Fig. 2, and 44 in Fig. 3;

Figs. 5 and 7 are enlarged perspective views of electron flow control grids embodying the present invention which may be employed in the structure shown in Figs. l4;

Figs. 6 and 8, respectively, are sectional views taken substantially along the line 66 in Fig. 5, and the line 88 in Fig. 7;

Figs. 9 and 10 are graphical representations of measured characteristics of the X-ray generator tube shown in Figs. l-4;

. Fig. 11 is a schematic view of a power supply system for the operation of the X-ray generator tube; and Fig. 12 is a graphical view of operating potential relationships which may prevail in the tube when the same is in operation.

To illustrate the invention, the drawings show an electron flow device 11 having an anode 12 and a cathode 13, enclosed in a hermetic envelope 14. The device shown comprises an X-ray tube adapted to produce an X-ray beam 15 at and to project the same from the anode 1 2, outwardly of the envelope 1 4, as through a suitable X-ray window 16 formed in the envelope, in response to impingement, upon a target surface 17 of theanode, of electron comprising a beam 18 emitted bythe cathode and impelled toward the anode target surface, under the driving force of a suitable electrical potential applied between the anode and the cathode.

V The anode 12 may comprise a body or block 19 of material, such as copper, forming an anode head, a .stem 20 of electrical conducting material carrying the 'head 19 within the envelope 14 and extending outwardly of the envelope at an end thereof for connection in a suitable external electrical power supply system, the envelope being sealed, as at 21, around said stern, and the head being provided with skirt means 22 enclosing the seal. A target button 23 forming the target surface 17 may be set in an end surface of the head 19 facing toward .the electron emitting cathode 13.

, The cathode 13 may comprise an electron emitting :filament 24 adapted to be electrically energized or excited for electron emission, said filament being mounted and supported in a suitable cathode structure and thus carried, within the envelope 14, in position to emit electronstoward the target surface 17 of the anode. The cathode support structure may comprise a disk or plate 25, forming a mounting head. The disk may be provided with a pair of spaced channels 26 extending therethrough and a transverse groove 27 extending diametrally of the head and interconnecting the open ends of the channels 26, in the anode facing or front surface of the head. A pair ofinsulating conductor mountings 28 may be secured to the rear face of the head 25 at and in alinement with the channels 26. These insulatmg mountings may each supportconducting stems 29 .in axial alinementin the channels 26, the filam'ent24 being electrically connected at its opposite ends with-the conductor stems 29 and thereby supported, diametrally of the head, in the groove 27, the filament and its suptrons.

4 port stems being electrically insulated with respect to the head 25.

The stems 29 have ends projecting through the supports 28 and outwardly of the head 25, said ends being electrically and mechanically connected, as by soldering, brazing, or otherwise, respectively, each with a corresponding mounting stem 30 of electrical conducting material. These mounting stems may extend outwardly of the envelope 14, as through a suitable seal 31. The mounting stems 30 thus not only serve to electrically connect the conducting stems 29 and the filament 24 with an external power supply system, outwardly of the envelope, for energizing the electron emitting element 24, but said stems 30 also serve, in conjunction with the stems 29 and the insulating supports 28, to mechanically support the cathode structure and the emitting filament 24 on and within the envelope in alined relationship with respect to the anode for cooperation therewith in the production of the X-ray beam 15. The cathode structure may also include a skirt member 32, which may comprise a cylindrical shell, preferably of metal, surrounding the head 25, supports 28, stems 30 and seal 31, said cylindrical skirt being supportingly secured on the head 25, as by brazing or welding an end of the skirt element upon the marginal edges of the head.

The X-ray beam 15 may be established as the result of impingement of the electron beam 18 upon the target surface 17 by energizing the filament 24 for electron emission, as by applying filament exciting power between the conductors 30 outwardly of the envelope, and by applying electron driving potential between the anode and cathode, as by applying such potential, at suitable electrical pressure or voltage, between the anode stem 20 and one of the conductors 30, outwardly of the envelope.

In order to control the operation of the device 11 in accordance with the present invention, a control grid 33 is mounted within the envelope 14 in position disposed between the electron emitting element 24 and the target surface 17 of the anode. By suitably biasing the grid 33 with respect to the emission element 24, the flow of electrons comprising the beam 18 may be accurately regulated not only to control the rate of electron flow to position and for biasing the same, the grid conveniently may be mounted upon the head 25 of the cathode structure at the anode facing surface thereof, whereby the grid is disposed immediately in front of the emission element 24.

The grid should comprise material having a high melting temperature and a high work function, that is to say, the material should be highly reluctant to emit elec- Tungsten, platinum, and molybdenum are suitable grid materials, although where molybdenum is employed it is desirable to coat the same with platinum to minimize electron emission, as molybdenum has a work functionrelatively lower than that of platinum. The grid 'structure may comprise a frame member 34 of disk-like character, having a central, preferably rectangular, opening 35 therethrough. The frame preferably comprises electrical conducting material, such as a suitable metal, and carries grid bars 36 of suitable electrical conducting material of filamentary character and electrically connected therewith, said bars preferably being mounted on the frame 34 in spaced parallel relationship, in position extending transversely in the opening 35. As indicated more particularly in Figs. 5 and 6 of the drawings, the gridma'y be fabricated by forming the grid bars 36 as filamentary wires positioned in spaced relation upona face of the frame 34 and secured in place by means'of a layer of solder 37. Alternately, the grid may be formed as shown in Figs. 7 and 8, by pressing or punching openings in a plate of suitable material to thereby provide a grid comprising bars integral with the supporting plate.

Where the wires, in a structure of the sort shown in Figs. 5 and 6, comprise material different from that of the support plate, it is desirable to select wire of material having thermal expansion characteristics such that the wires will be held under at least slight tension to keep the same from bending or buckling in the opening 35, when the grid becomes heated.

The marginal portions of the disk may be formed in conformity with the peripheral shape of the head 25 in order to facilitate the mounting of the grid on the head. While any suitable means may be employed for so mounting the grid, it may be attached as shown by merely applying it upon the face of the head 25and securing it in position by means of inwardly flanged portions of the skirt element 32. In this connection, the skirt 32 may be formed with an integral end wall 38 provided with a central opening 39, whereby the grid element may be mounted and secured in the cathode structure as a part of the assembly of the skirt element therein. This may be accomplished merely by depositing the grid upon the head 25, then applying the skirt element 32 and securing the same to the head 25 in position with the end wall portions 38 of the skirt element overlying the frame of the grid, to thereby hold it tightly in position and in electrical contact with the head 25. If desired, of course, the grid may be mounted and secured in the skirt element in alined position at the opening 39, whereby the skirt and grid may be assembled as a unit upon and secured to the head 25 of the cathode structure. In either case the grid may bemounted on the head 25 with the grid wires 36 disposed in spaced relationship with respect to the front face of the head 25, to thereby accurately determine the distance between the grid wires and the electron emitting element 24. The grid, furthermore, is assembled in oriented position, such that the grid wires 36 extend each in a direction normal with respect to the axis of the emission element 24, as clearly indicated in Fig. 3 of the drawings.

It will be seen from the foregoing that the grid wires 36 will all be electrically connected with the head 25, through the frame 34, and will be disposed in the path of electrons emitted from the filament. In order to apply a bias on said Wires, from a source of biasing power outwardly of the envelope, for the control of the emitted electrons, a conductor stem 40 may be provided in electrical contact with the head 25, and extending thence outwardly of the envelope through the seal 31. As a consequence, the grid wires may be biased with respect to the filament by applying biasing power between the conductor 49 and one of the conductors 3t), outwardly of the envelope. 7

The distance between the grid wires 36 and the filament should be quite small, of the order of of an inch, and the spacing between the grid, the emitting element, and the focusing cup structure afforded by the grooved head 25 is indeed quite critical, the desirable relationship of the parts being as shown in Figs. 2 and 3, in which the relative dimensions are illustrated for an assembly in which the emitting filament 24 is approximately of an inch in length between the axes of the support stems 29.

By means of the grid 33 appropriately biased, the flow of electrons between anode and cathode can be elfectively cut off completely, as by applying a suitable negative biasing potential between the grid and the emitting element 24. Electron flow between cathode and anode may, of course, be varied by adjusting the cathode exciting current supplied to the filament 24, as in a conventional X-ray generating tube, or by adjustment of the biasing potential applied on the grid in accordance with the present invention, such characteristic of a sample tube embodying the invention being illustrated in Fig. 9 of the drawings.

The size of the focal spot area of the anode target, within which electron impingement is obtained, does not appreciably vary with changes in the degree of cathode excitation, as measured in filament exciting current, nor with changes in anode-cathode potential; but focal spot size varies as a function of grid bias, and hence may be changed by altering the grid bias potential, the characteristics in this respect of a sample tube embodying the present invention being illustrated in Fig. 10 of the drawings. Accordingly, variation in the grid Voltage may be employed simultaneously to vary focal spot width and tube current in proper ratio to the loading of the target, whereby to maintain such loading within commercially usable limits. As a consequence, focal spot sizes giving optimum detail in X-ray shadow pictures made with rays generated in accordance with the present invention, are automatically achieved. Electron distribution upon the focal spot throughout the operating range of the tube is substantially uniform.

The ability afforded by the present invention, of obtaining control of the X-ray generator tube merely by controlling the low current grid voltage, allows considerable simplification in auxiliary equipment employed for powering and controlling the operation of generator tubes embodying the present invention, the automatic maintenance of focal spot size and tube current in proper relationship with focal spot loading being an obvious advantage inherently providedas overload protection, thereby avoiding the necessity of providing external or auxiliary overload protection means heretofore considered necessary in the operation of conventional generator tubes.

As shown more particularly in Fig. 11, conventional means may be employed for supplying an operating voltage between the anode and cathode of the generator tube 11. As shown, a transformer 41 may be employed, the secondary winding 42 of the transformer 41 being connected between the anode stem 20 and one of the cathode conductors 30, the primary winding 43 of the transformer being energized from any suitable electrical power source of alternating or fluctuating character. Conventional means may also be employed for supplying energizing power for cathode excitation. transformer 44 may be employed, the secondary winding 45 of the transformer 44 being connected between the conductors 30, the primary winding 46 of the transformer being energized from any suitable electrical power source of alternating or fluctuating character.

For the purpose of applying a bias potential on the grid 33, power may be supplied from any suitable source for application between one of the cathode energizing conductors 30 and the grid energizing conductor 40. To this end, an insulating transformer 47 may be employed, said transformer having a secondary winding 48 and a primary winding 49 which may be connected with a source of biasing power of alternating or fluctuating character. The secondary winding may be connected in series with a grid leak 50, comprising a condenser 51 and a leak resistor 52 in parallel relationship between one side of the secondary winding 48 and the grid conductor 40, the remote side of the secondary winding 48 being connected with one of the cathode conductors 30.

In the circuit arrangement shown in Fig. 11, an alternating or fluctuating voltage, approximately out of phase with the anode voltage, may be applied to the grid 33 through the resistor condenser network 50, the grid circuit being insulated by virtue of the insulating character of the transformer 47. The relationship of the voltages that may thus be applied is illustrated graphically in Fig. 12 of the drawings, from which it will be seen that, when the grid side of the insulation transformer is relatively positive, current will flow in the grid circuit as electrons are drawn from the filament 24 to the grid 33. Such current flow causes a voltage drop across the resistor 52, thus charging the condenser 51 to the peak value of the voltage drop. As the voltage As shown, a

of the transformer 47 diminishes, the condenser 51 will discharge through the resistor in accordance with the equation ly negligibly small; R is the value of the resistance 52 in ohms; C is the value of the capacitance 51 in farads; and T is the time in seconds.

The voltage thus applied between the grid and the cathode is a summation of the output voltage of the insulation transformer voltage and the voltage across the network 50. It should be noted that this voltage will be very. slightly positive during the negative half cycles of the operating voltage applied between anode and cathode. At all other times the biasing voltage will be negative and will eflectively apply a desired negative bias voltage during the positive half cycle intervals of anodecathode voltage, during which such anode-cathode voltage tends to drive electrons from cathode 13 to anode 12. The bias voltage thus applied to the grid may be used either to cut off electron flow between cathode and anode completely, or may be adjusted to permit any desired degree of electron flow merely by varying the voltage applied to the primary winding 49 of the insulation transformer 47.

The bias voltage can not under any circumstances become positive during the positive half cycle periods of anode-cathode voltage unless the values of the condenser 51 and of the resistance 52 are improperly selected to produce an unduly small product of R C, or unless the insulation transformer voltage is not substantially 180 out of phase with respect to anode-cathode voltage. On the other hand, the value of R C must not be too large for the reason that, if such value is too large, an objectionably long time interval will be required to adjust the electron flow between cathode and anode, that is to say, tube current. Suitable values of resistance and capacity for the network 50 respectively are 47,000 ohms and 0.01 lfarads.

The foregoing circuit arrangement has the advantage of aflording simple, inexpensive and highly effective means for controlling the bias voltage on the grid 33, and thus controlling the operation of the X-ray generator tube, without providing for the alteration of either filament excitation or anode-cathode operating voltage. Furthermore, if the values of the condenser 51 and of the resistor 52 are carefully chosen, the desirable phase relationship between anodecathode voltage and the voltage of the insulation transformer need not be held critically in 180 opposition, but the phase relationship may vary within a range of as much as :30", that is to say, from 150 to2l0.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. A control system for an X-ray generating tube having an anode and a cathode structure, comprising a head formed with a groove in a face thereof, an elongated electron emitting element supported on and insulated from said head in position extending in said groove, and an electron flow controlling grid comprising filaments supported on said head in parallel spaced relationship extending transversely of said groove, and biasing means for applying a biasing potential of alternating character between said emitting element and said grid, said biasing means including means for adding a unidirectional voltage component to said biasing potential to render asymmetrical the voltage values of alternate half cycles of said biasing potential.

2. A control system for an X-ray generating tube having an anode, an electron emitting cathode, and a control grid, comprising means for applying an operating electron driving potential of alternating character between the cathode and anode of the tube, and biasing means for applying a biasing potential of alternating character between said cathode and grid and displaced as to phase with respect to said operating potential, said biasing means including means for adding a unidirectional voltage component to said biasing potential to render asymmetrical the voltage values of alternate half cycles of said biasing potential, whereby the applied bias potential may be of negative character during the positive half cycle periods of said operating potential.

3. A control system for an X-ray generating tube having an anode, an electron emitting cathode, and a control grid, comprising means for applying an operating electron driving potential of alternating character between the cathode and the anode of the tube, and biasing means for applying a biasing potential of alternating character between said cathode and grid and displaced as to phase with respect to said operating potential, said biasing means comprising a grid leak circuit embodying a resistance-capacity network, and a source of alternating current power interconnected between said emitting element and said grid.

4. A control system for an X-ray generating tube having an anode, an electron emitting cathode, and a control grid, comprising means for applying an operating electron driving potential of alternating character be tween the cathode and the anode of the tube, and biasing means for applying a biasing potential of alternating character between said cathode and grid and displaced as to phase with respect to said operating potential, said biasing means comprising a grid leak circuit embodying a resistance-capacity network and the secondary winding of an insulation transformer having a primary winding energizable from a source of alternating electrical power.

5. A control system for an X-ray generating tube having an anode, an electron emitting cathode, and a control grid, comprising means for applying an operating electron driving potential of alternating character between the cathode and the anode of the tube, and biasing means for applying a biasing potential of alternating character between said cathode and grid and displaced as to phase with respect to said operating potential, said biasing means comprising a grid leak circuit embodying a resistance-capacity network and the secondary winding of an insulation transformer having a primary winding energizable from a source of alternating electrical power, and means for adjusting the potential applied on said primary winding from said power source.

6. A control system for an X-ray generating tube having an anode providing an electron target surface, a cathode embodying an electron focusing head and an electron emission element supported on and electrically insulated from said head, means for applying an operating electron driving potential of fluctuating character between the emission element and the anode of said tube, said control system comprising a grid mounted on said head between the emission element and the anode, means for applying a fluctuating potential upon said grid to bias the same with respect to the emission element in accordance with the variant intensity of said fluctuating electron driving potential, and means for adding a unidirectional voltage component to said fluctuating potential biasing said grid.

7. A control system for an X-ray generating tube having an anode, an electron emitting cathode, and a control grid, comprising means for applying an operating electron driving potential of alternating character between the cathode and the anode of the tube, biasing means for applying a biasing potential of alternating character between said cathode and grid and displaced as to phase with respect to said operating potential, and means for adding a unidirectional voltage component to the biasing potential to render asymmetrical the voltage values of alternate half cycles of said biasing potential, said control grid being disposed in the electron flow path between the cathode and the anode and being spaced from the cathode a distance less than /a of the average distance of the cathode from the anode.

8. A control system for an X-ray generating tube having an anode, an electron emitting cathode, and a control grid, comprising means for applying an operating electron driving potential of alternating character between the cathode and the anode of the tube, biasing means for applying a biasing potential of alternating character between said cathode and grid and displaced as to phase with respect to said operating potential, and means for adding a unidirectional voltage component to the biasing potential to render asymmetrical the voltage values of alternate half cycles of said biasing potential, said control grid being disposed in the electron flow path between the cathode and the anode and spaced not more than /8 inch from the cathode.

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